Although I have a 2012 8.5DS, I went through a similar dilemma when I picked it up late last year.

From my experience, disc brakes will lock up if you squeeze the lever hard enough. I've done it a couple of times but they are easy to modulate once you get use to it.

The H2 (28 and 35) and H5 (32, 42 and 45), the H4 is unavailable, come in different widths, you should be fine with any of those widths on your rims. This year I am staying on paved roads so I switched the Bontrager LT3 tires to a set of Continental Sport Contact Reflex 700x37 tires, much faster! If you want to dabble in some light off-roading, you can put a slick/semi-slick on the rear and a more aggressive tire on the front.

Wheels are a good improvement, but lighter, stronger wheels are expensive. You can change those yourself if you are familiar with lacing the spokes and truing the rim.

As far as other improvements, you can always swap out some stock components for lighter components - stem, saddle, seat tube, handlebars. A more comfortable saddle may improve your riding experience. If your commute is generally flat, how about a smaller cassette? Clipless peddles are always fun, but you'll also need a pair of corresponding shoes.